It is known in the prior art that the breaking resistance qualities of a rigid thermoplastic with respect to an impact are enhanced by the addition of, by blending and/or mixing, a proportion of an additive having an elastomeric component with the rigid thermoplastic. See, U.S. Pat. No. 2,992,203, Protzman, issued July 11, 1961 entitled "High Impact Resistant Polymers Derived Primarily From Esters of Acrylic and Methacrylic Acids." The prior art includes "elastomeric" additives intended for use in polyvinylchloride plastics, such as described by Ryan et al. in U.S. Pat. No. 3,448,173 issued on June 3, 1969 entitled "Acrylic Modifiers Which Impart Impact Resistance and Transparency to Vinyl Chloride Polymers," and such "elastomeric" additives intended for use in acrylic plastics, such as described by Griffin et al. in U.S. Pat. No. 3,450,796, issued on June 17, 1979 entitled "Production of Thermoplastic Polymeric Materials."
The addition of an "elastomeric" additive to a rigid acrylic thermoplastic has posed some problems which are not normally encountered with polyvinylchloride plastics. Thus, it is known that when an elastomer per se is added to a rigid acrylic thermoplastic: (1) transparency may be adversely affected; and the blended plastic may be susceptible to crazing upon exposure to (2) weather and (3) detergents.
Efforts to develop impact additives which enhance breaking resistance and avoid such adverse effects are described in the abandoned patent application of Ennor et al., Ser. No. 241,486 filed in the U.S. Patent Office on Dec. 23, 1962 entitled "Polymeric Moulding Compositions" and the abandoned patent application of Griffin, Ser. No. 293,194 filed in the U.S. Patent Office on July 5, 1963 entitled "Production of Thermoplastic Polymeric Materials." Bild, et al. refer to both applications in U.S. Pat. No. 3,234,303 issued Feb. 8, 1966 entitled "Continuous Process for the Production of Methacrylate Polymers."
Impact additives of this type are generally discrete, consecutively polymerized particles, including layers of an elastomer and a rigid thermoplastic. E.g., one such additive consists of an emulsion polymerized cross-linked acrylic elastomer, usually a lower alkyl acrylate, to which a rigid acrylic polymer shell, i.e., methyl methacrylate (or co-polymer), is then consecutively polymerized. Such layered particles are then mixed or blended with a rigid thermoplastic acrylic molding powder, (usually a co-polymer of polymethyl methacrylate (PMMA) and polyethyl acrylate (PEA) in the conventional commercial proportion of 96/4::PMMA/PEA). Optimally, a uniform and intimate admixture of the impact resistance enhancing additive particles with the acrylic thermoplastic composition should be achieved.
Variations of this type of impact additive including several consecutively polymerized layers or shells are described by Owens in U.S. Pat. No. 3,793,402 issued Feb. 19, 1974 entitled "Low Haze Impact Resistant Composition Containing A Multi-Stage Sequentially Produced Polymer" and U.S. Pat. No. 3,808,180, issued Apr. 30, 1974 entitled "Composite Interpolymer and Low Haze Impact Resistant Thermoplastic Compositions Thereof." Both of these patents and Owens' U.S. Pat. No. 3,843,753, issued Oct. 22, 1974 entitled "Composite Interpolymer and Low Haze Impact Resistant Thermoplastic Compositions Thereof" show blends of such "layered" or "shelled" impact resistance enhancing additive particles with acrylic thermoplastic materials.
The use of layered consecutively polymerized elastomer containing particles remains today a predominant method for enhancing the impact resistance properties of a rigid thermoplastic. See Hofmann, U.S. Pat. No. 4,180,529, issued Dec. 25, 1979 entitled "Acrylic Multistage Graft Copolymer Products and Processes."
The reason that layered latex particles which contain an elastomeric component in a layered/consecutively polymerized relationship with respect to a thermoplastic must first be made, and then blended, with a rigid thermoplastic, is that an elastomer per se is unmanageable. It is necessary to produce discrete particles with a final compatible "hard" thermoplastic polymer surrounding the elastomer (and other possible layers) so that the resulting particle, containing the elastomer to be added is processable, susceptable of handling, and compatible for blending. The final "hard" polymer shell may be graft-linked to the elastomer or to the preceding "shell." The art has recognized that such layered particles also facilitate the uniform dispersion of the elastomer throughout an acrylic thermoplastic material. [See Hwa et al., U.S. Pat. No. 3,661,994 issued May 9, 1972 entitled "Graft Polymers of Rubber for Reinforcing Plastics."]
In the above-referred patents naming Owens as inventor, it is suggested that composite elastomer/thermoplastic layered particles may be included in a casting mix or syrup which is then used to prepare, by bulk polymerization, a rigid thermoplastic blend having enhanced impact resisting properties. [E.g., Owens, U.S. Pat. No. 3,793,402, supra, Column 10, lines 30 et. seq.] The bulk casting produced consists of a dispersion or suspension of composite layered particles containing an elastomer, distributed throughout the bulk cast rigid thermoplastic. The casting may then be granulated and processed in an extruder, [Id., Column 10, lines 55 et seq.], and the resulting product is a "blend" which has enhanced impact resisting properties.
Similarly, Bild et al. in U.S. Pat. No. 3,234,303, supra, also describe the addition of layered consecutively polymerized emulsion/suspension polymerized particles containing an elastomeric core or subsequent shell (such as described in the abandoned applications filed in the U.S. Patent Office of Ennor et al., Ser. No. 241,486, supra, and Griffin, Ser. No. 293,194. supra) into an extruder apparatus employed in a continuous polymerization process to produce an extrudable acrylic polymer in which the additive particles are intimately blended with a principal thermoplastic component. As Bild et al. relate, if the additive is available as a slurry or latex, there is no need to dry off the water if an extruder having a devolatilization zone is used. [U.S. Pat. No. 3,234,303, supra, Column 5, lines 20 et. seq.].
In U.S. Pat. No. 3,796,677, issued Mar. 12, 1974, entitled "Incorporating Rubber Into Thermoplastic" Laber et al. disclose that an elastomer latex including a volume of absorbed water (i.e. a moist latex) may be mixed with a thermoplastic melt and the water thereafter removed by evaporation.
Such prior art methods for the integration of an elastomer into a rigid thermoplastic to provide enhanced impact resisting properties in the resulting product include undesirable process features. Thus, for example, when layered additive particles are produced in a consecutive emulsion process, it may be necessary to dewater and purify the particles. [E.g. Laber et al., U.S. Pat. No. 3,796,677, supra, Column 2, line 41 et seq.]. The criticality of particle size for certain applications (e.g. a large uniform particle size is required for transparency and low haze levels) further makes the maintenance of process parameters more difficult during production. The production of elastomeric latex particles, per se, also presents similar problems.
Purification and size control may require steps of coagulation, precipitation, spray drying, washing, filtration, dewatering and the like. There remains the possibility that impurities from an emulsion or suspension process will remain in the blend and adversely affect its properties. In addition, steps such as spray drying or coagulation may present environmental disadvantages in the disposal of water and/or calcium chloride used in a coagulation process. In any event, when a layered elastomer containing particle is produced by consecutive polymerizations in a latex process and the particle is used as an impact additive for a rigid thermoplastic (by blending the layered particles with the thermoplastic), there are possibilities that impurities remaining from the emulsion process or variation in particle size will remain and adversely affect the blended product.